Scanning Probe Microscopy of Polymers - American Chemical Society

Chapter 13

Downloaded by GEORGE MASON UNIV on July 7, 2014 | http://pubs.acs.org Publication Date: July 7, 1998 | doi: 10.1021/bk-1998-0694.ch013

Optical and Structural Properties of LangmuirBlodgett Films at the Air-Water and the Air-Solid Interface L. M. Eng Institute of Quantun Electronics, E T H Hönggerberg, HPF E7, CH-8093 Zürich, Switzerland

We report on novel investigations of non-linear optical 2docosylamino-5-nitropyridine (DCANP) Langmuir-Blodgett (LB) films by means of polarisation, second harmonic and scanning force microscopy (SFM). Direct optical measurements at the air/water interface showed that the molecules reoriented when depositing a L B film onto a solid substrate by the Langmuir-Blodgett method. Using the Langmuir-Schäfer method for horizontal dipping, however, resulted in the same surface structure being transferred onto substrates as was found for the D C A N P film floating on a water subphase. Furthermore, the D C A N P L B film was improved in film quality by adding 10-20% of arachidic acid (AA) to the pure D C A N P phase. As shown by S F M , this results in a smooth surface structure free of defects while no significant changes in the optical properties of the mixed L B film were found. Such L B films therefore show improved structural and optical properties for optical wave guide applications. Additionally, a novel force microscope was set up which is able to image the conformation of the chromophores of a L B monolayer floating at the air/water interface. Combined operation of both this new force microscope and the optical methods offers the possibility for the simultaneous inspection of both the structural and optical properties of the floating Langmuir films down to the molecular scale.

©1998 American Chemical Society

In Scanning Probe Microscopy of Polymers; Ratner, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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Introduction For the application of non-linear optical materials in wave guide structures, filters and switches, an essential prerequisite is the overall homogeneous and constant properties of the material in use. It is not only necessary that the non-linear optical material has a high hyperpolarisation, but also that its physical structure is smooth on a scale much smaller than the typical wavelength. Recently, we showed that scattering losses at defects of 10-100 nm in diameter significantly attenuate the intensity of the propagating electromagnetic wave (-20 dB/cm) [1]. Thus we are interested in improving the material quality for the assembling of a usable wave guide structure. In a bottom-up approach the construction of a perfect monomolecular layer is desired using L B molecules. Not only do our organic films offer a big variety for chromophore syntheses, but also a wave guide structure can be controlled on a molecular level. Here, we investigated the effect of L B film transfer onto a solid substrate, a crucial problem with these type of molecular structures. The influence of asymmetric forces on the molecules can be seen in direct space using our new set-up of second harmonic (SH) and polarisation microscopy [2J. Furthermore, film quality of freely floating Langmuir films was controlled using a novel S F M specially designed for the inspection of liquid/air interfaces.

Results and Discussion Fig. 1 presents the D C A N P monolayer film structure (a) when floating on the water subphase on a L B trough, (b) deposited onto a Pyrex substrate by perpendicular dipping (LB method), and (c) horizontally dipped by the Langmuir-Schafer (LS) method. Note that in the LS method, the horizontally mounted sample substrate slightly touches the floating L B molecules until they interact with the atoms at the sample surface. Therefore, the structure of the Langmuir films transferred by the LS method is quite similar to the floating monolayer (see Fig. l a and lc). In both cases the D C A N P film appears in so-called spherulites of a mixed crystalline and amorphous structure [3]. Perpendicular dipping (Fig. lb) results in preferential arrangement of the chromophore molecules along the dipping direction. This is due to the force gradient induced by capillary and meniscus forces at the water/substrate interface [3]. The induced non-linear optical anisotropy measured parallel and perpendicular to the dipping direction is 10:1. Fig. 2 presents the optical set-up as used to measure the reorientation of the D C A N P L B molecules when deposited onto solid substrates. As seen the molecules are randomly oriented when floating on the water subphase (left). However, when interaction between the molecular dipoles of the chromophores and the surface atoms occurs, the molecules change their conformation and arrange along the dipping direction (right).



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Figure 1. Domain structure of a D C A N P L B film (a) floating on the water subphase, (b) deposited perpendicular onto a Pyrex substrate by the L B method, and (c) horizontally dipped by the LS method.

Figure 2. Direct imaging of the reorientation of L B molecules when being deposited onto a solid substrate. While the molecules are randomly oriented when floating on the water subphase (left part), elongated domains appear on the solid substrate (right part) arranged parallel to the direction of dipping.


226 The mechanical film properties of the D C A N P L B film were further improved by adding a known percentage of arachidic acid (AA) to the D C A N P phase [4]. Fig. 3 shows two S F M images representing (a) the pure D C A N P L B film and (b) the mixed D C A N P / A A structure containing 20% of A A in the D C A N P phase. While the pure D C A N P film (Fig. 3a) undergoes a structural phase transition with the L B film shrinking laterally by 10-15%, the mixed L B film presents a smooth sample surface topography containing only a minor number of defects. The latter sample is observed to be stable for several weeks and the rms surface roughness being constant at 0.1 nmrms measured over 1 p m . We therefore succeeded in optimising a L B film with respect to its structure and transferring it onto a solid substrate.


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Fig. 4 illustrates the behaviour of the non-linear optical susceptibility as a function of A A concentration. As seen from this figure, the non-linear optical intensity is not affected too much for A A concentrations smaller than 20%. For higher concentrations, S F M inspection shows that the mixed L B film segregates into the two components forming islands of A A molecules in the D C A N P phase. The phase separated film has a lower optical signal since the A A islands induce scattering losses of a propagating electromagnetic wave, in the same way as did structural defects in the pure D C A N P film (see Fig. 2a). The optical intensity drops to zero for A A concentrations bigger than 30 %.

To complement our optical techniques (polarisation and second-harmonic microscopy) which were already set up on top of the Langmuir-Blodgett trough we constructed an underwater scanning force microscope (SAFM) [5] which is able to image the structural conformation of L B molecules floating at the liquid/air interface, i.e. a Langmuir layer. The S A F M was constructed as a standalone, remote-controlled and water-tight scanning force microscope [5]. Fig. 5 shows a constant height S F M picture of the freely floating A A Langmuir film. To perform this experiment A A was chosen as a L B molecule since it is known to exhibit a very well ordered and solid phase. A A molecules were spread onto the water subphase (containing 10 mol of CdCl for stabilisation) and then compressed to 30 mN/m. As seen from Fig. 5 the A A headgroups are well aligned along distinct directions with the most pronounced of them crossing the picture diagonally from the bottom-left to the top-right corner. The unit cell formed by the headgroups of the floating A A film was measured to 0.5 x 0.5 nm and agrees well with the values found for A A films transferred to solid substrates. -3

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The results reported above clearly show that S F M can equally be used for the inspection of liquid/solid interfaces as well as liquid/air interfaces. With the invention of the S A F M it is possible to answer some important scientific questions which are relevant for molecular binding and agglomeration processes of individual molecules at this interface. Because van der Waals, electrostatic, solvation and steric interaction forces between the floating Langmuir layer and the S F M tip are essentially the same as those acting between adjacent headgroups within a layer, we should expect to find many correlations between interlayer and intralayer forces. Fig. 6 finally presents a series of force - distance (F-s) curves recorded with the S A F M at various interfaces. While Fig. 6a) presents the reference curve recorded



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Figure 3. (a) 3.7 x 3.7 p m S F M image of the D C A N P L B film deposited onto a mica substrate. Ageing results in film shrinking of 1015%. The molecular height between dark and grey levels measures 2.1 nm. (b) 2 x 2 p m S F M image of the mixed D C A N P / A A monolayer. At 20% A A in the D C A N P phase, the surface has a very smooth appearance showing very few defects. 2

AA concentration [%]

Figure 4. Non-linear optical susceptibility d$3 of a mixed D C A N P / A A L B film as a function of A A concentration.



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Figure 5. S A F M picture of the freely floating AA monolayer taken over an image size of 10 x 10 nm . The headgroups of the A A molecules are aligned along specific directions indicating good short range ordering. 2



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a •s Q &

fa

10 20 30 tip/sample distance

[nm]

Figure 6. Force - distance (F-s) curves recorded with the S A F M at various interfaces (k = 0.64 N/m) : (a) polycrystalline gold sample, (b) floating A A L B film, and (c) floating D C A N P L B film. Note the similarities in F-s curves between Au and A A .


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on a polycrystalline gold sample, Fig. 6b) and Fig. 6c) illustrate the behaviour for two floating L B films : a densely packed A A L B film and a fluid-like D C A N P L B film, respectively. A l l curves show the repulsive force interaction exerted between tip and sample. While the z-values are accurately read and represented in a nm-scale (horizontal axes), force numbers are not calibrated (vertical axes). Nevertheless, a relative comparison between the three F-s curves is possible : First, both the A A and the Au curve are very similar in shape. In fact, the A A L B film was compressed into its solid phase in order to deduce the lateral packing of molecules with the S A F M (see Fig. 5). The only difference stems from the force values recorded for A A and Au, which are smaller by a factor of 10 when comparing Fig 6b) with Fig. 6a). Recording the same curves with a cantilever of a very soft spring constant (k = 0.03 N/m) instead of k = 0.64 N/m (not shown [6]), the two force plots become even more similar with the force values deviating only by a factor of 2. A floating A A L B film therefore may be regarded as forming a sold interface with a very high elastic modulus in the same way as does a solid substrate. Secondly, comparison of Fig. 6b) and Fig. 6c) shows the pronounced difference between the solid- and the liquid-like L B film. While force values deviate by a factor of 2-3 only (even for a strong cantilever spring constant k = 0.64 N/m) interaction forces are found to be longer ranged extending over more than 30 nm. Clearly the D C A N P L B film contains rather strong dipolar headgroups which significantly influence the force interaction between tip and sample. We therefore expect the S A F M to be sensitive enough to record both dipolar and van der Waals interactions in floating L B films.

Acknowledgement We are pleased to express our gratitude to Ch. Seuret, P. Gunter, and F. Griinfeld. Financial support by the Swiss National Science Foundation under grant # 30520 is greatly acknowledged.

References 1. Ch. Bosshard, M. Küpfer, M. Flörsheimer, and P. Günter, Thin Solid Films 1992, 210/211,153. 2. M . Flörsheimer, D. Jundt, H . Looser, K. Sutter, M. Küpfer, and P. Günter, Ber. Bunsenges. Phys. Chem. 1994, 98, 521. 3. L . M . Eng, M. Küpfer, Ch. Seuret, and P. Günter, Helv. Phys. Acta 1994, 67, 757. 4. B . Schmidt, Kraftmikroskopische Untersuchungen an DCANP und VECANP Langmuir-Blodgett Filmen, Diploma thesis, ETH Hönggerberg, Zürich, Switzerland; 1993. 5. L . M . Eng, Ch. Seuret, H. Looser, and P. Günter, J. Vac. Sci. Technol B 1996, March/April, 1386. 6. Ch. Seuret and L . M . Eng, to be published.


Scanning Probe Microscopy of Polymers - American Chemical Society

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